1. Field of the Invention
The present invention relates to a solar cell composed of a nitride semiconductor.
2. Description of the Related Art
Recently, a semiconductor device composed of a nitride semiconductor such as gallium nitride (GaN) has been researched and developed actively. A semiconductor optical device made of nitride semiconductors such as aluminum nitride (AlN), gallium nitride (GaN), indium nitride (InN) or mixed crystal thereof absorbs light in a wide wavelength region from ultraviolet or blue to infrared by varying its film composition. The nitride semiconductor has a property of direct transition, and the light absorption coefficient thereof is as high as 104 to 105. Furthermore, compared with silicon (Si), which is a material widely used as a semiconductor material for a solar cell, the light absorption coefficient of the nitride semiconductor is more than one-hundred times higher than that of silicon. For this reason, as its application, a solar cell using a nitride semiconductor has been proposed (for example, see Omkar Jani et. al., Appl. Phys. Lett. Vol. 91 (2007)132117).
FIG. 11 shows a cross-sectional view of a solar cell according to prior art. As shown in FIG. 11, the conventional solar cell has a low-temperature grown buffer layer 102 consisting of GaN, an n-type GaN layer 103, a light absorption layer 104 consisting of InxGa1-xN, a p-type GaN layer 105, and a p-type contact layer 106 consisting of p-type GaN, all of which are formed on a sapphire substrate having a main surface with a (0001) surface orientation. A p-side electrode 107 is formed on the p-type contact layer 106. An n-side electrode 108 is formed on the selectively exposed region of on the n-type GaN layer 103.
According to the prior art, in order to suppress carrier recombination due to non-luminescence transition caused by lattice defects or threading dislocations, it is necessary to prepare a nitride semiconductor film having significantly low defects in the crystal. For this reason, a single-crystalline substrate such as a sapphire substrate has been used. Such a single-crystalline substrate, however, is very expensive.
In order to solve the above-mentioned problem, a method for preparing a polycrystalline nitride semiconductor film on a graphite substrate by a pulse sputtering method has been proposed (For example, see Japanese Laid-open patent publication No. 2009-200207).